Numerical focusing of ultrasound

The main principles and restrictions of the numerical focusing such as the acoustical holography and the synthetic aperture focusing technique (SAFT) are described. In the holography the ultimate lateral resolution is half of the wavelength. The longitudinal resolution is poor. With SAFT the longitudinal resolution is equal to pulse length. The transversal resolution is half of the wavelength or half of the diameter of the probe. The single frequency holography is optimal to image planar reflectors. When planar reflectors are inclined with respect to the aperture they can best be imaged with SAFT or multifrequency holography. Some images based on the inspections with SAFT carried out by IzfP in Saarbrcken are shown. The specimens to be tested were supplied by the Technical Research Centre of Finland (VTT). The results were in good agreement with earlier measurements with surface wave method